1. Field of the Invention
The present invention relates to a magnetic disk system constructed from a magnetic head assembly and a magnetic disk unit, and more particularly to a magnetic disk system having a magnetic disk unit which has a feature which permits at least one magnetic head to be positioned on a selected data track accurately without the necessity of forming servo information on the disk and a magnetic head assembly which is suitable for its alignment with a track (i.e. tracking) and its azimuth alignment.
2. Description of the Related Art
In general, with a magnetic disk unit, it is required to accurately position a magnetic head on data tracks of a magnetic disk at the time of reading of information from or writing of information on the disk. Heretofore, tracking servo systems for positioning a head on tracks include the following systems.
(1) The servo-surface servo system which uses a dedicated servo surface.
(2) The data-surface servo system which uses servo information recorded on a data surface.
(3) The servo-surface and data-surface combined system which uses both a servo surface and a data surface in combination.
In the first place, with the servo-surface servo system, servo information is formed over the whole of a certain surface (e.g. servo surface) of more than one disk surface and the servo information is continuously read to permit a servo head on the servo surface to track servo tracks correctly. A data head, which is mounted on the same carriage as the servo head and makes access to another disk surface (e.g. data surface), interlocks the head on the servo surface to make access to a data track corresponding to a servo track. With the servo-surface servo system, when expansion and contraction of the disks occur due to variations in temperature and humidity, a problem arises in that the head will go off a data track (i.e. thermal off-track) depending on a difference in expansion and contraction between disk surfaces. In addition, the servo-surface servo system uses the whole of a surface of a disk as the servo surface and thus formatting efficiency will be decreased particularly when the number of disks is small.
With the sector servo system which is a type of the data surface servo system, servo information is formed on a part of sectors, i.e., a servo sector. The so-called sampled value control system for tracking is used which permits the servo information to be read by the head which makes access to a data surface for reading or writing. With the sector servo system, since the servo information is demodulated for each of the data surfaces to position a corresponding head on a selected track, such thermal off-track as described above will never occur. According to the method, the tracking servo is performed by the sample value control because the servo information is read intermittently for each of servo sectors. Compared with the above-described servo-surface servo system in which servo signals are continuously obtained, therefore, a wide servo band cannot be achieved. The trackability will be poor if it is assumed that there is no thermal off-track. In the sector servo system, the settling-time is longer and the disturbance-proof performance is also poorer.
With the servo surface and data surface combined system, on the other hand, the servo information is formed on each data surface with the servo head positioned on a servo track on the servo surface, whereas, in the servo-surface servo system, data is read from or written on a data surface by the data head with the servo head positioned on a track on the servo surface. The servo information is formed on a part of servo sectors, i.e., the servo sector. From the servo information formed in the servo sector on the data surface is obtained a position signal (information indicating the position of the head relative to a track) which is in a low frequency range and contains a DC (Direct Current) component which is used to perform tracking control. In the combined system as well, the format efficiency is poor and the circuitry used is complex in construction. Because there is the necessity of servo formatting as well as data formatting, a problem will arise in that it takes a long time to establish and implement the system.
The demand for higher recording density in magnetic recording/reproducing apparatus such as magnetic disk units has been increasing recently. For a higher recording density, the track density and linear density must be increased. Narrowing the width of tracks will also be required. As a magnetic recording medium use has been made of not only a longitudinal recording medium on which information is recorded using the usual recording method but also a perpendicular recording medium and an obliquely evaporated/sputtered recording medium. Under such conditions, it has been required that the registration of a read/write head with tracks on a magnetic recording medium and the azimuth alignment be performed more precisely. In order to detect track offset and azimuth offset necessary for the registration and azimuth alignment of a head, a specific type of magnetic head has been used.
FIG. 36 illustrates an example of a conventional magnetic head which uses two induction type heads to detect track offset. That is, magnetic cores 101 and 102 consisting of a soft magnetic material are disposed side by side in the direction of the width (indicated by W) of a track and coils 103 and 104 are wound around the magnetic cores 101 and 102, respectively. Supposing outputs of the induction type heads to be V1 and V2, respectively, when the head positions right over the track, the following equation will hold. EQU V1-V2=0
When the head does not position right over the track, the following relationship will hold. EQU V1-V2.noteq.0
Thus, the track offset is detected from the difference magnitude between V1 and V2 and the azimuth can be detected from the difference in phase between V1 and V2.
With the arrangement of FIG. 36 in which two inductive-type heads each having a magnetic core wound with a coil are disposed side by side, however, a space must be secured to permit the coils to be wound around the cores and the dimension of the heads in the direction of width of a track must be made large. Thus, the conventional head shown in FIG. 36 will have a structure which is not suited to narrow the tracks of a disk.
As a magnetic head which needs no coil, on the other hand, there is an MR head which uses Magnetoresistance (MR) film having the magnetoresistance effect. FIG. 37 illustrates which is referred to as a yoke type MR head which uses a yoke (i.e., magnetic core) to conduct leakage flux from a magnetic recording medium to an MR film. The MR head is constructed from a lower core 201, separated upper cores 203 and 204 disposed above the lower core with a read/write gap 202 disposed therebetween, an MR film 205 disposed between the upper cores 203 and 204 and electrodes 206 and 207 connected to the both ends of the MR film 205.
FIGS. 38A and 38B illustrate conventional shield type MR heads having a shielding function by an MR film. As shown in FIG. 38A, between that ends of magnetic cores 208 and 209 which form a recording/reproducing gap are disposed tips of an MR film 205 and electrodes 208 and 209. In FIG. 38B, between a magnetic core 208 disposed below a magnetic core 209 and a newly provided shielding plate 200 are disposed an MR film 205 and electrodes 206 and 207. In the case of FIG. 38B, a recording gap is formed between ends of the magnetic cores 208 and 209, while a reproducing gap is formed between the magnetic core 208 and the shield plate 200.
Of the conventional tracking servo systems for placing a head on tracks, the servo-surface servo system using a dedicated servo surface has the off-track problem (a) in which the head cannot be placed on a selected track correctly depending on a difference in expansion and contraction between disk surfaces which is due to variations in temperature and humidity, and the poor formatting efficiency problem (b) arising from the fact that the whole of a surface of a disk is used as a servo surface only.
Of the data-surface servo systems, the sector servo system in particular has not the off-track problem but problems (c) in that not only the trackability is poor because the servo band cannot be made high by the use of the sampled value control system but also the settling-time is longer and the disturbance-proof performance is poorer.
The servo-surface and data-surface combined system is poor in format efficiency and complex in circuit arrangement. Moreover, the problem (d) arises in that it takes a long time to establish and implement the system because servo formatting must be performed on a data surface in addition to data formatting.
In the conventional magnetic head assembly, as described above, two inductive-type heads are disposed adjacent to each other in the direction of the width of a track in order to detect track offset and azimuth offset. With the magnetic head assembly of such a structure, however, the problem (e) arises in that its structure becomes unsuitable for narrowing tracks because its dimension along the width of a track has to be made large because of the necessity of securing a space for winding coils.